ci: add Python 3.14 coverage and align tested-version metadata #350

Problem and evidence

At main f5e7c49554ff02771990a151e00b52f2abf3df0a, core CI runs Python 3.9/3.13 on Ubuntu/macOS and 3.13 on Windows. Optional MCP uses 3.10/3.13 and Relay uses 3.11/3.13. No Python 3.14 cell is present. Package classifiers end at 3.13, while requires-python >=3.9 and the documented support contract permit newer Python.

The v1.3.2 incident record describes Direct pairing/probes on Homebrew Python 3.14.9. That bounded manual evidence does not provide repeatable CI coverage of core, packaging or optional runtimes.

Expected behavior and impact

Continuously validate Python 3.14 rather than relying on isolated manual results. This closes a test-coverage gap; no Python 3.14 runtime defect or incompatibility is claimed.

Scope / acceptance checks

• Add Python 3.14 to the Ubuntu/macOS core matrix, retaining existing minimum-version and Windows coverage.
• Evaluate and add 3.14 coverage for the supported MCP/Relay profiles, or explicitly document any dependency/support blocker instead of claiming coverage.
• Run generated-standalone, relevant subsystem and packaging/install checks on the new interpreter; keep the aggregate release gate meaningful.
• Add the 3.14 classifier only after its intended supported scope passes, and align the support documentation.
• Confirm existing Python floors (core 3.9, MCP 3.10, Relay 3.11) and minimum/latest MCP dependency-profile coverage remain intact.
• Record exact CI evidence; distinguish fixture tests from native-agent/platform acceptance.

Review limits

Read-only source and open/closed issue/PR review on 2026-10-29; no existing Python 3.14 matrix issue was found. No local test, dependency install or native experiment was run. The external review's Python 3.15.2/test-count claim is not used as independent validation.